RHEOLOGICAL IMAGES OF POLY(VINYL CHLORIDE) GELS - 5 - EFFECT OF MOLECULAR-WEIGHT DISTRIBUTION

Three blends were prepared from a high molecular weight of poly(vinyl chloride) (PVC) (M-w = 173 000, M-w/M-n = 2.0) and a low molecular wei ght PVC (M-w = 39 400, M-w/M-n = 1.7). Dynamic viscoelastic properties of these PVC blends in bis(2-ethylhexyl) phthalate (DOP) were measure d at 40 degrees C as a function of polymer concentration, and the effe ct of long chains on gelation has been studied. The scaling exponent n at the gel point was found to be constant (=0.75), independent of mol ecular weight and molecular weight distribution. The critical concentr ation c(g) for the sol-gel transition still followed the relation c(g) proportional to M-w(-1), which was unchangeable with the molecular we ight distribution and was also in good agreement with the previous res ults. As a result, c(g) was well expressed by a mixing rule, 1/c(g) = w(1)/c(g1) + w(2)/c(g2), where w(i) is the weight fraction of the comp onent polymer i. The gel strength S-g at the gel point did not obey th e relation S-g proportional to M-w(-1), but scaled as S-g proportional to M-z(-1) to show the effect of long chains on gelation. In the post gel state, the gel elasticity determined by the quasi-equilibrium modu lus G(e) still followed the scaling law, G(e) proportional to epsilon( z), where epsilon is the relative distance to the gel point and z = 2. 6 for the (PVC blend)/DOP samples, but the G(e) values at the same eps ilon were observed to be dominated by the long chains of PVC.